This invention relates to optical fiber cables and, more particularly, to optical fiber cables comprising a central buffer tube containing one or more optical fibers, reinforced by longitudinal strength members to protect the optical fibers from forces, such as installation and thermally induced expansion and contraction of the buffer tubes.
Optical fibers are relatively fragile and must be protected during the manufacture, handling and installation of cables including such fibers. A variety of protective measures are therefore provided in cables containing optical fibers. For example, to allow the cable to move or be flexed a certain degree by external forces or by thermal expansion and contraction without stressing or microbending the optical fibers, the optical fiber or fibers are typically enclosed in a plastic buffer tube having a bore of a cross-sectional area larger than the cross-sectional area of the fiber or fibers within it. This is referred to as a xe2x80x9cloosexe2x80x9d configuration. The material of the tube typically has a relatively high temperature coefficient of expansion and a relatively low tensile strength. Frequently, the length of the optical fibers is greater than the length of the tube, referred to as excess fiber length. An optical fiber cable may include one or several buffer tubes, each containing one or a plurality of optical fibers. The plurality of optical fibers may be in the form of individual fibers, an optical fiber ribbon or a stack of optical fiber ribbons, for example.
To further resist thermal expansion and contraction, as well as longitudinal loads which may be applied during manufacture, handling and installation of the optical fiber cable, strength members of metal wires or high strength non-metallic rods or fibers, such as glass rods or fibers may be embedded in the material of the buffer tube. See, for example, U.K. Patent Application GB 2,215,081, which provides two diametrically opposed wires or high tensile plastic filaments entirely within the walls of the buffer tubes. U.S. Pat. No. 4,898,451 also shows a plurality of flexible fibers of aramid, steel or glass, completely encircled by the plastic of the buffer tubes contained within an optical fiber cable.
Strength members have also been provided in grooves in the outer surface of the buffer tubes. See, for example, U.S. Pat. Nos. 4,804,245 and 4,610,505. In both patents, the strength members are held in the grooves by an outer wrapping. In the ""245 patent, the shape of the grooves and the physical properties of the strength members are such that there is little resistance to compression forces. Also, the optical fibers are not encircled by, or centrally disposed with respect to the buffer tube. The ""505 patent relies on the wrapping to provide coupling between the strength members and the tube and the optical fiber is not loosely contained in a buffer tube. The requirement of such a wrapping complicates the structure of the cable and could increase its diameter.
In an alternative configuration disclosed in U.S. Pat. No. 5,509,097 (xe2x80x9cthe ""097 patentxe2x80x9d), assigned to the assignee of the present invention and incorporated by reference herein, an optical fiber cable is disclosed including a buffer tube loosely enclosing optical fibers. Two diametrically opposed, longitudinally extending strength members are tightly coupled to the buffer tube by an adhesive or a cord to resist tensile and compression forces applied to the buffer tube and prevent buckling of the strength members. The core of the cable can also include a layer of extruded plastic which presses the strength members toward the tube to increase the coupling between the strength members and the buffer tube and which opposes buckling and kinking of the strength members.
When the strength members are at the outside of the buffer tube as shown in the ""097 patent, the structure is not circular in cross-section. If the structure shown in the ""097 patent is to be covered by other layers, the outer surface may have xe2x80x9clumpsxe2x80x9d at the location of the strength member, and it may be desirable to have an outer surface corresponding substantially to the outer surface of the cylinder. Furthermore, if the strength members are entirely outside the outer surface of the buffer tube, special apparatus is required in applying a plastic layer over the combination of the buffer tube and the strength members, and the diameter if the jacket may be increased. If the diameter of the layer is increased, the diameter of the cable is increased which is undesirable. If the layer is omitted, it becomes difficult to apply the armoring, etc., because the strength members, being at the outer surface of the buffer tube, constitute protrusions and deviations from a cylindrical surface to which the armoring, etc., must conform. It can be desirable, in some cases, to omit the layer and to minimize the extent to which the strength members extend outwardly from the buffer tube without losing the benefits of the structure shown in the ""097 patent and particularly, the tight coupling of the strength members to the buffer tube.
If the strength members are completely encircled by a substantial amount of the buffer tube material, it is difficult to main the strength members in the correct positions within the buffer tube, and it can be necessary to increase the thickness of the buffer tube wall because of the presence of the strength members therein which increases the amount of buffer tube material required and increases the diameter of the buffer tube and hence, the diameter of an armored cable incorporating such structure. Completely embedded strength members can also interfere with gaining midspan access to the optical fibers during splicing because the location of the strength member is not visible. Further, attempting to remove these strength members necessarily damages the structural integrity of the tube, increasing the risk of damage (i.e. kinking, crushing) to the optical fibers as the midspan operation continues. Examples of cables with completely embedded strength members have been described hereinbefore
It is known to include water blocking compound within the buffer tubes for moisture protection. The water blocking compound, which permits the optical fibers to move within the buffer tube, may be a gel or grease-like, and non-hygroscopic and/or thixotropic. Any spaces between the buffer tube and the outer jacket can also be filled with a water blocking compound. Further moisture protection is provided by water blocking yarn and/or water blocking tape disposed between the buffer tube and the outer jacket. Such yarn or tape may be provided within the buffer tube instead of the water blocking compound. See, for example, U.S. Pat. No. 5,071,221.
Additional layers of materials, such as armoring for crushing and rodent protection, can also be provided between the buffer tube and outer jacket. Longitudinal ripcords are typically provided to assist in opening the optical fiber cable jacket and armor, if provided.
In a telecommunications network, certain optical fibers within an optical fiber cable may be spliced to other optical fibers, such as optical fibers leading to particular terminal locations. The optical fibers may also need to be accessed to check for faults. To achieve midspan access to the particular optical fibers to be spliced or checked at a particular location along the cable, the jacket and the buffer tube containing the optical fibers and any intervening layers must be opened. Typically, the optical fiber cable is opened by accessing and pulling the ripcord to open the jacket and armor if present, and cutting through or removing other layers of material, such as water blocking tape, to expose the buffer tube of interest.
When strength members are embedded in the cable jacket, they are not tightly coupled to the buffer tube and the strength members in the region of the jacket to be opened must be removed before the jacket and armor can be opened. A short section of the jacket material directly over those strength members is first removed by shaving with a sheath knife. The strength members are then pried away from the jacket to expose the underlying layers. Since the length of the cable section to be opened, which usually is 8-10 feet in length, it is necessary that a corresponding length of the strength members also be removed from this section. Such removal of the strength members is a difficult and labor intensive operation. After the buffer tube is exposed, the buffer tube may be slit in a known fashion to access the optical fibers contained therein. The optical fiber cables can be secured within a splice enclosure during the midspan access and splicing procedure, as is known in the art. The splice enclosure is used to seal the cable when the process is completed. Accordingly, while strength members in the jacket can be used, it is desirable that the strength members be directly coupled to the buffer tube.
An optical fiber cable with strength members tightly coupled to the buffer tube which resist thermal expansion and contraction of the buffer tube, as well as tensile forces, wherein the strength members are readily apparent and do not interfere with gaining midspan access and which has reduced manufacturing problems without increasing the cable diameter, would be advantageous.
In accordance with one embodiment of the present invention, an optical fiber cable core comprises a buffer tube of a plastic material loosely containing and completely encircling at least one optical fiber. The buffer tube has a longitudinal axis, a nominal outer periphery, and has at least two strength members tightly coupled to the buffer tube and with surface portions thereof proximate the nominal outer periphery of the buffer tube. If the strength members have surface portions outwardly of the nominal periphery of the buffer tube, lip portions of buffer tube material extend towards each other and engage the strength members. The strength members extend longitudinally of the core and substantially parallel to the longitudinal axis of the buffer tube. Thus, each strength member is partially embedded in the buffer tube such that the lip portions extend over and engage a portion of the outer surface of the strength members. Buffer tube material partially surrounding and engaging the strength members, including the lip portions, applies radially inward forces the strength members, thereby tightly coupling the strength members to the buffer tube material, to provide resistance to contraction and expansion forces, as described in the ""097 patent. The radially outermost surface portion of each strength member, i.e. in the radial direction with respect to the longitudinal axis of the buffer tube, may be substantially tangent to or protrude beyond the outer surface of the buffer tube such that less than half of the outer surface of each of the strength members protrudes beyond the nominal outer periphery of the buffer tube. If portions of the outer surface of the strength members protrude outwardly of the nominal periphery of the buffer tube, longitudinal portions of one or both of the strength members can be easily pulled away from the buffer tube by pushing the buffer tube material adjacent to the strength members, e.g. the lips, away from the strength members by one""s fingers without the use of tools. Removal of one or both of the strength members enables midspan access to the optical fiber or fibers within the buffer tube without cutting the strength members or damaging the structural integrity of the tube. Also, since the strength members are visible, they can be avoided during slitting of the buffer tube by positioning the blade of the slitting tool between the strength members. Therefore, removal of the strength members may not be required. Of course, if it is desired to cut the strength members during splicing of the optical fibers, that can be easily done, as well.
In order to provide the desired coupling between the strength members and the buffer tube, the material of the buffer tube must apply pressure to the strength members in the direction radially of the axis of the strength member. Thus, the resistance to slipping between the strength member and the buffer tube depends on the force directed by the material of the buffer tube radially toward the axis of the strength member, the coefficient of friction of the materials of the strength members and the buffer tube and the extent of the surface of the strength member to which the forces are applied.
Preferably, in order to provide the desired coupling, more than 50% of the surface of the strength members is contacted by material of the buffer tube, but exept possibly for thin film of the buffer tube material which does not prevent identification of the location of the strength members, which applies insignificant forces to the strength members and which overlies the radially outermost, with respect to the buffer tube axis, surface portions of the strength members, the strength members are not entirely surrounded by buffer tube material.
Preferably, less than about 20% and more preferably, less than about 10%, of the outer surface of each of the strength members protrudes beyond the nominal outer periphery of the buffer tube. However, due to manufacturing tolerances, a thin film of buffer tube material, which can easily be removed and which does not obscure the strength members, may cover the outermost surface portions of the strength members. The film has a thickness which is small compared to the wall thickness of the buffer tube wall. With or without the film, the strength members can be readily located and the desired coupling between the strength members and the buffer tube is obtained.
By so coupling the strength members to the buffer tube, the resistance of the strength members to compression forces is improved because the buffer tube resists bending and kinking of the strength members described in the ""097 patent. Thus, although the methods described in the ""097 patent, e.g. adhesive or a cord, can also be used with the embodiments of the invention to resist bending and kinking and thereby increase the compression resistance of strength members of a small diameter, the coupling of the strength members to the buffer tube provided by the invention can be sufficient, by itself, to provide the desired resistance to bending and kinking especially if the strength-member-buffer tube structure is encircled by an extruded layer of plastic material contacting the buffer tube and the strength members.
Preferably, two diametrically opposed strength members are provided. The strength members can comprise metallic or non-metallic materials which are sufficiently incompressible in the direction radially of the axis of the strength member to permit the desired coupling of the strength members to the buffer tube. For example, the strength members can be metal or glass rods, but cannot be compressible fibers, e.g. fibers which are merely twisted or stranded together. To increase the coupling between the strength members and the buffer tube, the strength members can be coated with a material which bonds to the strength members and which will bond to the buffer tube during extrusion of the buffer tube material, e.g. an adhesive or material of the buffer tube.
Preferably, the buffer tube contains a plurality of optical fibers in the form of a plurality of optical fiber ribbons forming an optical fiber stack, which is completely enclosed by a wall of the buffer tube. Preferably, water blocking material is provided within the buffer tube.
If it is necessary to accommodate the strength members, e.g. if the diameter of the strength members is such that the buffer tube wall is weakened, the buffer tube of the present invention may have thicker walls and have a larger diameter than conventional buffer tubes. This could prevent the use of conventional slitting tools to gain midspan access, such conventional tools being designed to slit buffer tubes having a relatively small wall thickness. In such case, the buffer tube can be made with two layers, an inner tubular portion and an outer tubular portion, each with a radial dimension which can be slit by a conventional tool. With two such layers, preferably, the inner tubular portion is coated, prior to extrusion of the outer tubular portion over the coated inner portion, with a material which permits the inner and outer portions to be pulled apart easily in a direction perpendicular to their adjacent surfaces, but which does not significantly reduce the resistance of the two portions to slippage relative to each other in the direction longitudinally of the tubes. Portions of the outer tubular portion can then be easily removed from the inner tubular portion, which may be of a conventional thickness and diameter, enabling the use of conventional slitting tools to gain access to the optical fibers through the inner tubular portion.
In accordance with another embodiment of the present invention, an optical fiber cable comprises the optical fiber core described hereinbefore, and at least an outer jacket encircling the buffer tube and strength members. Preferably, a water blocking tape or water absorbing yarn is provided between the jacket and buffer tube. Preferably, armor shielding is provided between the jacket and the water blocking tape, if present. If no water blocking tape is provided, the armor engages both the jacket and the buffer tube.
Preferably, the space within the buffer tube not occupied by the optical fibers or optical ribbon, are filled with a water blocking material, such as a gel, which is substantially incompressible and which resists radially inward forces applied to the buffer tube and hence, the strength members. In this embodiment, radially inward forces applied by the jacket, or by a cord as described in the ""097 patent, increases the coupling between the strength members and the buffer tube.
In accordance with a preferred embodiment of the invention, a reinforced buffer tube, which can be used in an optical fiber cable, is manufactured by hot extruding a buffer tube with longitudinal strength members proximate, tangent to or protruding from the nominal outer periphery of the tube and cooling the extruded tube, causing contraction of the buffer tube material. The buffer tube, which usually is made of plastic, is formed with a material having a coefficient of contraction greater than the coefficient of contraction of the strength members. The buffer tube wall and the wall of the strength members are selected with respect to coefficients of contraction so that when the buffer tube is cooled after the heated buffer tube wall is applied, the buffer tube material contracts and grips the strength members, applying sufficient radially directed forces to the strength members to prevent longitudinal slippage of the strength members with respect to the buffer tube under normal field conditions. Lip portions of buffer tube material extending towards each other and engaging the strength members, can be formed proximate radially outermost surface portions of each of said strength members, to further couple the strength members to the buffer tube material as the buffer tube material contracts during cooling. When so manufactured, less than half of the outer surface of the strength members protrude beyond the nominal outer periphery of the buffer tube. Radially outermost surface portions of the strength members are substantially tangent to or protrude outwardly from the buffer tube.
The manufacture of the reinforced buffer tube can include first extruding an inner tubular portion and then applying the strength members while extruding, with heating, an outer tubular portion over the inner tubular portion. The thickness of the outer tubular portion is selected so that the strength members are positioned within the outer tubular portion with portions of the outer surface of the strength members closely adjacent to an outer surface of the outer tubular portion. As the outer tubular portion cools it contracts, thereby positioning the strength members substantially tangent to or protruding beyond the nominal outer periphery of the buffer tube, as described above, and becoming tightly coupled to the outer and inner tubular portions. As described hereinbefore, prior to extrusion of the outer tubular portion, the inner tubular portion may be coated with a material which enables easy removal of the outer tubular portion from the inner tubular portion and yet resists longitudinal slipping between the inner and outer tubes so that conventional slitting tools can be used to gain midspan access to the optical fibers through the inner tubular portion.